The present invention relates to a hydraulic brake system with brake slip control and a device for active braking, including a master cylinder which is connected to a supply reservoir, at least one wheel brake which is connected to the master cylinder by way of a brake line, a return pump which is connected to the wheel brake by way of a first suction line and to a precharging device by way of a second suction line, wherein a means is arranged in the second suction line that controls the pressure fluid supply to the suction side of the return pump.
A brake system of this type is disclosed in German patent applciation No. 40 25 859, for example. The prior art brake system exhibits a front-axle/rear-axle brake circuit split-up, and one brake circuit is associated with a non-driven axle and the other brake circuit is associated with a driven axle. The brake circuit of the driven axle is equipped with a device for traction slip control. Traction slip control is an active braking mode because braking intervention is carried out without application of the brake pedal. Further ways of active braking are, for example, braking interventions for yaw torque control without participation of the driver.
In brake slip control operations, the prior art brake system operates according to the recirculation principle and includes a precharging pump for the return pump for traction slip control purposes. The precharging pump is connected with its suction side to the supply reservoir of the brake system and with its pressure side, by way of a hydraulically operated on-off valve, to the suction side of the return pump. When the supply pressure of the return pump reaches the maximum pressure which is provided in the brake system during traction slip control operations, the delivered pressure fluid flows through a pressure-limiting valve and propagates into the control line for the hydraulically operated on-off valve between the precharging pump and the return pump. In the further course of traction slip control, a precharging volume is not delivered to the return pump until the pressure in the control line has decreased to such an extent that the on-off valve reopens. The purpose of this provision is to prevent the delivery of unnecessary pressure fluid volume by the return pump. The objective is that the return pump produces pressure only when it is actually required. Thus, instead of discharging a superfluously delivered fluid volume by way of a pressure-relief valve into the supply reservoir of the brake system, the suction side of the return pump is shut off until fluid delivery is needed again.
Other possibilities of precharging a return pump, no matter whether it is of the self-priming or non-self-priming type, are provided by using an active vacuum brake force booster, an additional pressure accumulator or, for example, a precharging device between the supply reservoir and the master cylinder so that the passage through the master cylinder into the brake line is used to precharge the return pump when the brake pedal is not applied.
However, the return pump in all these constructions is greatly stressed by the precharging device in the active braking mode and is subjected to premature wear. Major leakages will then cause failure of the electric motors of the pumps in the extreme case. A leak-proof design of the electric motors would involve considerable effort and structure, however.
Therefore, an object of the present invention is to provide a hydraulic brake system of the above-mentioned type where the wear of the return pump is reduced.
This object is achieved by controlling the pressure fluid supply as a function of the pressure gradient in the second suction line of the return pump.
The principle of the present invention in active braking involves preventing from the very start the pressure on the suction side of the return pump from rising to an extent which might cause damage. Such prevention is effected by controlling the pressure fluid supply during precharging as a function of the pressure gradient between the precharging source and the suction side of the return pump. Thus, the supply pressure of the return pump is no more decisive for a reduction of the pressure fluid supply.
A solenoid valve with a hydraulically insertable throttle is especially appropriate to limit the pressure fluid flow to the suction side of the pump.
The throttle may be configured so that, when the solenoid valve opens, it stays effective as long as a pressure gradient prevails between the precharging source and the return pump. When the precharging action is terminated, as is the case, for example, after some time when an active brake force booster is used, the throttle can be switched to open so that a larger valve passage opens. Then it is possible to replenish pressure fluid in a self-priming return pump by way of a large opening cross-section of the valve.
The idea of the present invention will be explained in detail by way of the description of an embodiment of the present invention in two Figures.